Polycrystalline materials serve as the basic building blocks for a vast assortment of man-made articles. The properties and behaviors of these materials are determined, at least in part, by the size and shape of the constituent crystallites or grains, the orientation of their crystal lattices, and how the grains are placed to fill space. Accordingly, these attributes of the materials microstructure must be determined in order to understand why certain materials behave as they do, to predict how materials will behave, and to alter or otherwise control material forming and processing techniques to improve specific material properties.
Automated orientation imaging microscopy (OIM) has enabled researchers, material processors, and manufacturers to obtain much valuable microstructure information over a relatively large material sample area. Generally, OIM repetitively bombards selected points of a material sample with a beam of electrons. The electrons interact with a small volume of the material sample at the selected points, and backscatter diffraction patterns form on a phosphor screen near the specimen and may be imaged through a video camera. The video images are called electron backscatter diffraction patterns (EBSPs) or backscatter Kikuchi diffraction (BKD) patterns.
Good quality, high contrast, EBSPs include a number of intersecting, relatively high intensity "Kikuchi" bands generally bordered by thin dark lines. The Kikuchi bands result from electrons being diffracted from various planes in the crystal lattice at the point of bombardment. An abundance of microstructure information, including lattice orientation, may be obtained by analyzing the various parameters of the Kikuchi bands. Sophisticated computer-implemented image processing techniques have been developed to analyze Kikuchi bands from EBSPs taken at numerous points on a material sample and to combine this information into OIM maps which describe a wealth of microstructure information.
Unfortunately, each EBSP may include hundreds of thousands of pixels, and tens of thousands of points on a single material sample may be bombarded with an electron beam to produce tens of thousands of EBSPs. Consequently, an immense number of computer operations must be performed to form a single OIM map. Even with very fast computers, the entire process of forming and analyzing each OIM map takes an undesirably long period of time.
A defect in a material is a relatively small region of a material sample which exhibits microstructure properties that differ from the properties exhibited throughout a larger region near the small region. Thus, examples of defects include grain boundaries, cracks, scratches, voids, plastic deformation, fatigue damage, and the like. In materials science and engineering, defects and the microstructure surrounding defects are often of much interest. Electron bombardment at a defect often yields a low quality EBSP from which Kikuchi bands may be detected with only a low degree of confidence. Consequently, defect information on conventional OIM maps is indirectly inferred from the absence of high confidence microstructure information at particular coordinates on a material sample after an exhaustive and thorough OIM analysis has taken place.